1. Field of the Invention
The present invention relates to a semiconductor laser comprised of a plurality of laser diodes having different oscillation wavelengths formed on an identical substrate and a method of producing the same, more particularly relates to a semiconductor laser having a reflection film for controlling a laser output formed on an end of each laser diode and a method of producing the same.
2. Description of the Related Art
As optical disc-shaped recording media for recording and/or reproducing information by emitting light (hereinafter referred to as optical discs), for example, compact discs (CD), Mini Discs (MD), digital versatile discs (DVD), etc. are irradiated with lights of different wavelengths in accordance with the type of the optical discs. For example, light of a wavelength of the 780 nm band is used for reproduction of data from a CD, while light of a wavelength of the 650 nm band is used for reproduction of data from a DVD.
An optical recording and/or reproducing apparatus able to handle different kinds of optical discs requires a plurality of light sources having different oscillation wavelengths. An optical recording and/or reproducing apparatus normally uses a laser diode as a light source, however, when forming a plurality of laser diodes, it becomes difficult to make the apparatus compact and the process of production becomes complex as well.
To overcome the above disadvantages, a multiple wavelength monolithic semiconductor laser formed with a plurality of laser diodes having different oscillation wavelengths on a single substrate has been developed.
Generally, semiconductor lasers are roughly divided into an end emission type laser for emitting laser light in parallel to an active layer, and a surface radiating type (surface emission type) laser.
A surface emission type laser Is capable of performing single mode oscillation and able to be used for long distance transmission, high speed transmission optical fiber communication etc., so the surface emission type multiple wavelength laser has drawn attention as a light source for parallel optical communication.
On the other hand, a laser used for data-recording on and data-reproducing from an optical disc preferably has a plurality of longitudinal modes in the gain spectrum because even if there are a plurality of longitudinal modes, the spatial coherence is not particularly deteriorated and because of the noise problem occurred when light is reflected from the disc and returns to the laser. An end emission type laser has a resonator overwhelmingly longer than the wavelength in a crystal and a large number of resonance modes in the resonator. Therefore, the end emission type laser is suitable as an optical pickup for CDs, MDs, DVDs, and other optical discs.
The configuration of a semiconductor laser of an end emission type will be explained with reference to FIG. 5.
As shown in the perspective view of FIG. 5A, an n-cladding layer 102 comprised for example of n-AlGaAs, a pn junction (active layer) 103 comprised for example of GaAs, and a p-cladding layer 104 comprised for example of p-AlGaAs are successively stacked on a substrate 101 comprised for example of n-GaAs. On the surface of the p-cladding layer 104 except for a striped area at the center is formed a high resistance layer 105. At an upper layer of the p-cladding layer 104 or the high resistance layer 105 is formed a p-electrode 106.
The high resistance layer 105 is formed by ion implantation of n-type impurities in the surface of the p-cladding layer 104. The striped area sandwiched between the parts of the high resistance layer 105 is left as a low resistance layer. By selectively forming the high resistance layer 105, the result is a gain waveguide structure (current constricting structure) as shown in the top view of FIG. 5B. It becomes possible to control the area in which the current flows, that is, the area where an optical gain is generated.
According to the laser of the above configuration, a resonator is formed in the active layer 103. As shown in FIG. 5B, although laser light 1 is emitted from a front end F, it is partially lost from the rear end R. The two ends of the emission area (optical waveguide path) 107, that is, the front end F and the rear end R, are mirror surfaces.
In order to make the ends mirror surfaces, a semiconductor wafer is normally cleaved. Alternatively, the mirror surfaces are sometimes formed by etching instead of cleaving. Also, dielectric films are sometimes formed on the cleaved facets in order to control the reflectance of the ends and to prevent the deterioration of the cleaved facts.
As the dielectric films formed on th ends, single-layer films of for example Al3O3, amorphous silicon, SiO2, or Si3N4 or multi-layer films comprised of a stack of these films may b used. By changing the thicknesses of the dielectric films, the reflectances of the ends can be adjusted. By making the front end F a low reflectance for example less than 30% and the rear end R a high reflectance for example more than 50%, preferably more than 70%, high output laser light can be obtained. The energy conversion efficiency, the front/rear output ratio, etc. depends on the reflectance. Accordingly, the dielectric film controlling the reflectance of the end is one of the important design parameters of a semiconductor laser.
The thickness of the dielectric film formed on an end is, when the oscillation wavelength is xcex, normally designed based on xcex/2 or its odd multiple or xcex/4 or its odd multiple. For example, in FIG. 5B, when forming a dielectric film 108 on the front end F by using Al2O3 having an oscillation wavelength xcex of 785 nm and a reflectance n1 of 1.62, the thickness d108 of the dielectric film 108 is determined as follows:
d108=(xcex/2)/n1xe2x89xa0242.3 (nm)xe2x80x83xe2x80x83(1)
Also, the rear end R has to have a high reflectance, however, when using the above Al2O3 etc. as a single layer, since the reflectance becomes less than 50% in any case, a plurality of dielectric films are formed. As shown in FIG. 5B, in the case of the oscillation wavelength xcex of 785 nm, when forming for example an Al2O3 film having as a first dielectric film 109a and an amorphous silicon film as a second dielectric film 109b, the thicknesses of the layers are determined for example as follows. The thickness d109a of an Al2O3 film having a reflectance n1 of 1.62 becomes
d109a=(xcex/4)/n1xe2x89xa0121.1 (nm)xe2x80x83xe2x80x83(2)
and the thickness d109b of an amorphous silicon film having a reflectance n2 of 3.25 becomes
d109b=(xcex/4)/n2xe2x89xa060.4 (nm)xe2x80x83xe2x80x83(3)
FIG. 6 is a graph showing the relationship of the thickness of the Al2O3 formed on the front end F and the reflectance of the front end F. FIG. 7 is a graph showing the relationship of the thickness of the Al2O3 film and amorphous silicon film formed on the rear end R with the reflectance of the rear end R. The oscillation wavelength xcex is assumed to be 785 nm in both FIG. 6 and FIG. 7.
As shown in FIG. 6, by making the thickness of the dielectric film of the front end F the above d108 and making the thickness of the dielectric film of the rear end R a combination of the above d109a, and d109b, the reflectances becomes the extremal values. Accordingly, it is possible to reduce the fluctuations in the reflectances accompanying variations in film formation.
By making th thicknesses of the dielectric films formed on the ends xcex/2 or its odd multiple or xcex/4 or its odd multiple or a combination of these, it becomes easy to obtain stable reflectances even when there is variation in the thicknesses or refractive indexes due to variation in the formation of the dielectric films.
In the case of a multiple wavelength monolithic semiconductor laser, ideally dielectric films are formed on the laser diodes having different oscillation wavelengths by the above design of the related art.
In this case, however, the production process becomes complex and the increase of the number of the production steps becomes a problem. When forming, for example, a CD playback laser diode and DVD playback laser diode on an identical substrate, an end of one, for example, the DVD laser diode (wavelength of 650 nm band) is masked and, in that state, a dielectric film is formed on an end of the CD laser diode (wavelength of 780 nm band).
To avoid the increase of the number of production steps as in the above case, there is the method of matching the optimal wavelength of the end coatings with on of the laser diodes and simultaneously forming end coatings on th plurality of laser diodes on the same substrate. In this case, however, while a reflectance stable against variations in film formation can be obtained for the laser diode of the design wavelength, the stability against variations in film formation of the reflectances of the other laser diodes is sacrificed.
The method has also been considered of reducing the fluctuations in the reflectances of the dielectric films of the ends of the plurality of laser diodes formed monolithically by calculating the optimal thickness for each of the laser diodes and forming in common dielectric films having thicknesses of the least common multiple of these values.
FIG. 8 is a graph showing examples of the cyclical fluctuation of reflectance when changing the thickness of a dielectric film. FIG. 8 shows the reflectances of a dielectric film comprised by Al2O3 with respect to laser light of a wavelength of 785 nm or 660 nm.
The optimal thickness of a dielectric film for laser light having a wavelength of 785 nm is 242.3 nm as defined in formula (1). From a similar calculation, the optimal thickness of a dielectric film for laser light having a wavelength of 440 nm is 203.7 nm. The reflectances for light having wavelengths of 785 nm and 660 nm become extremal at the least common multiple of the two thicknesses, that is, 1218 nm.
In this case, however, the dielectric film becomes extremely thick and the time for formation becomes long, so the production efficiency is low. Furthermore, due to the large thickness, the fluctuation in the reflectance becomes large when the variation of the formation becomes more pronounced.
An object of the present invention is to provide a semiconductor laser, formed with a plurality of active layers different in material and composition on an identical substrate and capable of emitting a plurality of laser light having different oscillation wavelengths, where dielectric films having little fluctuations in reflectance are formed at ends of the plurality of active layers.
Another object of the present invention is to provide a method of producing the above laser.
To achieve the above object, the semiconductor laser of the present invention is a semiconductor laser having a plurality of active layers of different composition on a substrate and emitting in parallel a plurality of laser light of different oscillation wavelengths, said laser comprising a front surface coating film formed on a front end of a laser emission side and a rear surface coating film formed on a rear end at a back side of said front end and having a higher reflectance compared with said front surface coating film and by thicknesses of said front surface coating film and said rear surface coating film being set to predetermined thicknesses giving reflectances of extremal values with respect to light of a predetermined wavelength between the minimum value and the maximum value of said oscillation wavelength.
Preferably, the predetermined wavelength is an arithmetical mean of oscillation wavelengths of said plurality of laser beams.
Preferably, the front surface coating film is comprised of a dielectric. Preferably, the rear surface coating film is comprised of a dielectric.
Preferably, the front surface coating film is comprised of a plurality of layers. Preferably, the rear surface coating film is comprised of a plurality of layers.
Preferably, the predetermined thickness of said front surface coating film is a value expressed by (xcex/2)/nF when said predetermined wavelength is xcex and the reflectance of said front surface coating film is nF.
Preferably, the rear surface coating film is comprised of stacked films of a first rear surface coating film having a reflectance nRa and a second rear surf ace coating film having a reflectance nRb; said predetermined thickness of said first rear surface coating film is a value expressed by (xcex/4)/nRa when said predetermined wavelength is xcex; and said predetermined thickness of said second rear surface coating film Is a value expressed by (xcex/4)/nRb when said predetermined wavelength is xcex.
Preferably, the active layer Is formed at an interlayer bonding portion of a first conductivity type cladding layer and a second conductivity type cladding layer.
Preferably, the active layer has a current constricting structure.
The reflectance of the end can be stabilized for each of the plurality of laser diodes having different oscillation wavelengths formed on the identical substrate. If forming a dielectric film having a thickness optimized with respect to the oscillation wavelength of the laser diode on an end of each laser diode, while a stable reflectance can be obtained even when there is variation in the thicknesses of the dielectric films, the production steps becomes complex. According to the semiconductor laser of the present invention, dielectric films of a common thickness are formed on the plurality of laser diodes. By suitably adjusting the thickness of the dielectric films, the reflectances at the ends of the plurality of laser diodes can be stabilized.
To attain the above object, the method of producing a semiconductor laser of the present invention is a method of producing a semiconductor laser for forming two laser diodes having different oscillation wavelengths on a substrate, characterized by including the steps of successively causing epitaxial growth of a first cladding layer, an active layer, and a second cladding layer for forming a first laser diode on a substrate to form a first stack; removing portions of said first stack other than said first laser diode; successively causing epitaxial growth of a first cladding layer, an active layer, and a second cladding layer for forming a second laser diode on a substrate to form a second stack; removing portions of said second stack other than said second laser diode and spatially separating said first stack and second stack; forming an electrode on a said laser diode; forming a front surface coating film having a predetermined thickness optimized with respect to a predetermined wavelength of an arithmetical mean of said oscillation wavelengths on an end of a laser emission side of a said laser diode; and forming a rear surface coating film having a predetermined thickness optimized with respect to said predetermined wavelength having a higher reflectance compared with said front surface coating film on an end of a back side of said laser emission side.
Preferably, the step of forming a front surface coating film and said step of forming a rear surface coating film are steps for forming films using a dielectric as a material.
According to the above method of producing a semiconductor laser of the present invention, the compositions of first and second stacks including the active layers can be made mutually different and a two-wavelength monolithic semiconductor laser can be formed by a simple process. Furthermore, since the front surface dielectric films or rear surface dielectric films are commonly formed on the two laser diodes, a semiconductor laser wherein the reflectances of the ends are prevented from fluctuation can be produced by a simple process.